I'm not sure about guitars, but for violins the effect is huge on the timbre and quality of sound. The Renaissance and early Baroque Italian luthiers spent much effort on perfect the f-shape that we now see everywhere today. Guitars may be another matter however; just thought that was useful info perhaps!
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NoldorinMay 27 '11 at 20:31

Currently we see round, oval, "f-hole", multiple/little, soundholes on the sides of the guitar, none whatsoever, etc, etc.
Much of the sound emanates from the entire vibrating surfaces of the guitar, including the back... There are several designs without any holes at all.

I think much of the variation we see is more in the name of style rather than function.

The hole itself doesn't affect the sound of the guitar. It is more the actual top (soundboard) of the guitar that generates the sound by vibrating like a drum skin. Usually, up to 80% of the sound is created from two vibration points to either side, and just below the bridge.

If you want to see which part of your soundboard is generating most of the sound - sprinkle some flour or fine sand all over the soundboard and pluck some open notes. You can then see the flour/sand settle into areas where there is no movement, and be bounced away from areas with lots of movement.

The trick is then not to damage or change the areas of maximum movement on the soundboard too much, as this will drastically affect the sound of the guitar.

Then again, if your guitar is TOO bright and you want to dampen the sound down a bit, you can stick bits of blu-tack or plasticine under the soundboard at these vibration points to dampen them a bit. Classical guitarsts do this quite often.

The trick is also getting all the flour/sand out of the guitar when finished. :)
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neilfeinApr 28 '11 at 2:44

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Haha - that is indeed a very tricky process Neil. Wouldn't recommend doing this to your expensive custom made guitar - although I DO know that John Williams has done this to his $25K Smallman classical guitar... :)
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DevanApr 28 '11 at 2:48

IS it possible to do something similar with the sides of a guitar? That could be extremely useful if considering cutting a hole in the side for a preamp; you'd find what areas to avoid. Or do the sides not contribute much to the guitar's tone?
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neilfeinApr 28 '11 at 4:23

Neil - you should be fairly safe with the sides - they don't vibrate too much - especially up closer to the neck.
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DevanApr 28 '11 at 12:04